From the world of Halo, a MAC (Magnetic Accelerator Cannon) gun is one of the humans defenses, usually placed on ships or orbital defense stations. Basically, a MAC gun is a similiar, if not the same, to a rail gun, with its high power and velocity/

A MAC gun is one of the UNSC's most powerful non-nuclear weapon. A MAC gun fires a ferrious core. The gun fires the 3,000 tonn projectile at 4/10th the speed of light and is the only known weapon to break through a covenant shield in one shot. MAC guns can be put on a UNSC ship or stationed above an important Earth colony